1. Field of the Invention
The present invention relates to an illuminating optical apparatus utilizing a coherent light beam such as an excimer laser beam, adapted for use in an exposure apparatus for semiconductor device manufacture or the like.
2. Related Background Art
As the light source for the exposure apparatus employed in the manufacture of integrated circuits, there have principally been employed ultra-high pressure mercury lamps. However, the recent remarkable progress in the level of integration of such integrated circuits is requiring an ever increasing precision on the line width. For this reason ultra-high pressure mercury lamps are starting to be replaced by high power lasers of shorter wavelength such as excimer lasers, as the light source.
The excimer lasers can be classified into two categories, one of which is called a stable resonator type. In this type, as shown in FIG. 1, a resonator is composed of a discharge lamp 100 for causing induced emission, with two resonator mirrors 102a, 102b positioned at opposite ends. The light reciprocates between the mirrors whereby the amplitude of the induced emission is intensified to emit a laser beam LB.sub.0. The laser beam emitted from such laser is characterized by low coherence in space and time. The low coherence in time signifies a wide half value width of the spectrum (.DELTA..lambda..congruent.0.4 nm). For use in the exposure apparatus for semiconductor device manufacture, such light source requires correction of color aberration in the projection lens, and it is difficult to provide a practical lens in such short wavelength range.
The other light source is called an injection lock type, which is divided into an oscillator and an amplifier as shown in FIG. 2. The oscillator is provided with oscillator mirrors 102a, 102b as in the above-mentioned stable resonator type. However, in the injection lock type, the oscillator is further provided with a wavelength selecting element 106 such as an etalon and a diffraction grating, and with apertures 104a, 104b at opposite ends of the discharge tube 100 for the purpose of intercepting the laser beam in a predetermined area, whereby the emitted laser beam has a narrower half-value width of the spectrum (.DELTA..lambda..congruent.0.001 nm) or improved monochromaticity. In addition the emitted laser beam is reflected by mirrors 108 and is amplified by unstable resonator mirrors 112a, 112b with mutually opposed convex and concave faces, positioned at opposite ends of a second discharge tube 110. One of the features of the laser beam from such laser is the improved monochromaticity with high coherence in time, whereby the correction of color aberration can be dispensed with in the projection lens.
For this reason the projection lens can be manufactured with a single material (quartz), so that the designing and manufacture are easy. However another feature of the injection locking laser is an extremely high coherence in space due to the amplification by the unstable resonator, and the use of such laser will result in marked interference fringes in the exposed area.
For avoiding such inconvenience, there has been developed a new type of laser as shown in FIG. 3, which is composed of the aforementioned stable resonator laser and a wavelength selecting element 114 for reducing the wavelength band width, such as an etalon, a prism or a diffraction grating, whereby the emitted laser beam has a narrower width of spectrum (.DELTA..lambda..congruent.0.003 nm). The laser beam from such laser has an improved coherence in time due to the presence of the wavelength selecting element 114, and a lower coherence in space in comparison with that from the injection locking type.
U.S. Pat. No. 4,619,508 discloses a method for reducing speckles resulting from the interference of a laser beam, by providing a rotating mirror or the like in the optical path of the illuminating system for causing two-dimensional scanning motion of the beam, thereby reducing the coherence in space.
Also U.S. patent application Ser. No. 135,378 filed Dec. 21, 1987 (now U.S. Pat. No. 4,851,978 issued Jul. 25, 1989) proposes effectively eliminating the speckles generated with the laser beam of high coherence in time and space emitted from the injection locking laser by means of beam vibration, i.e., two-dimensional movement of the beam in synchronization with the emitted pulses of the laser and in correspondence with the arrangement of elements of a lens member, such as a fly's eye lens, provided for obtaining uniform intensity distribution of the beam.
However, in such methods, it is difficult to obtain an appropriate amount of exposure within a short period, or the throughput has to be significantly reduced in order to secure the appropriate amount of exposure.
A high-power laser beam entering a fly's eye lens generates secondary light sources of an extremely high luminocity, which may cause destruction of optical components if such secondary light sources are focused thereon. Also weak reflected light, generated for example on a lens surface, if focused in the vicinity of the reticle, will be refocused on the wafer, thus generating a ghost image.
The first of the above-mentioned drawbacks of the fly's eye lens can be solved, as disclosed in the U.S. patent application Ser. No. 237,847 filed Aug. 26, 1988 (now U.S. Pat. No. 4,939,630 issued Jul. 3, 1990), by focusing the secondary light sources in a space outside the optical components. Also the latter of the drawbacks can be solved by a serial arrangement of two optical integrators, as disclosed in the U.S. Pat. No. 4,497,015.
However such double optical integrators give rise to the formation of speckles due to interference fringes. More specifically, the light fluxes emerging from the secondary light sources formed by the first optical integrator do not mutually interfere, but those from tertiary light sources formed by the second optical integrator interfere mutually for the following reason. The diameter of beams passing through the element lenses of the first integrator is expanded to about 10 times upon reaching the second integrator, so that the non-interfering distance of two light beams is also expanded to about 10 times. Thus the non-interfering distance becomes larger than the pitch of the element lenses of the second optical integrator, so that the tertiary light sources formed on the exit portions of the element lenses can mutually interfere.